A pacemaker is an implantable cardiac stimulation device for implant within a patient that analyzes an intracardiac electrogram (IEGM) to detect various arrhythmias, such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia), and then selectively delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An implantable cardioverter-defibrillator (ICD) additionally or alternatively detects atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation. Such devices are typically configured to be used in conjunction with an external programmer that enables a physician to program the operation of an implanted device to, for example, control the specific parameters by which the pacemaker detects arrhythmias and responds thereto. In particular, the physician may program a set of parameters that the device uses to automatically control the sensitivity with which the device senses electrical signals within the heart. The sensitivity determines the amplitude of signals to which the device's sense amplifiers will respond, typically specified in millivolts (mV). The higher the mV value, the lower the sensitivity. For example, if the sensitivity is set to 6 mV, a signal must be at least 6 mV before the signal will be recognized. If the sensitivity is only set to 2 mV, the signal need only be 2 mV before it is recognized. Hence, the lower the mV value, the more sensitive the device.
State-of-the-art pacemakers and ICDs are equipped with ASC, which is a procedure by which the device automatically adjusts atrial and ventricular sensitivity values during each individual cardiac cycle to keep the values at optimum levels. ASC and related techniques are discussed in U.S. Pat. No. 6,862,476 to Mouchawar, et al., entitled “Implantable Cardiac Stimulation Device Having Automatic Sensitivity Control and Method.” Current implementations of ASC are specified by several programmable parameters including: Starting Sensitivity, Decay Delay, Decay Rate, Maximum Sensitivity and Refractory Period, which depend upon whether ASC is applied to paced and/or sensed events and which further depend on whether ASC is applied to atrial and/or ventricular events. These parameters will now be described primarily with reference to a ventricular ASC example.
With ventricular ASC, the ventricular sensitivity is initially set to the Starting Sensitivity value immediately following delivery of a ventricular pacing pulse (i.e. a V-pulse) or detection of an R-wave (i.e. a ventricular depolarization event, also referred to as QRS-complex). The ventricular Starting Sensitivity is specified as a percentage of the average amplitude of prior R-waves and is typically set to a relatively high percentage value (e.g. 50%-90%) so as to provide a relatively high mV value (thus making the ventricular sense amplifier relatively insensitive to further signals at this time). The ventricular sensitivity remains at the Starting Sensitivity level for the duration of the ventricular Decay Delay interval. Separate Decay Delay values are used following sensed ventricular events (i.e. R-waves) and paced ventricular events (i.e. V-pulses.) Once the appropriate ventricular Decay Delay interval has elapsed, the device automatically and gradually decreases the ventricular sensitivity mV value in accordance with a ventricular Decay Rate (thus making the ventricular sense amplifier increasingly more sensitive to further signals.) The ventricular sensitivity continues to increase (i.e. the mV value continues to decrease) until the ventricular sensitivity reaches a ventricular Maximum Sensitivity (i.e. a minimum mV value) or until another R-wave is detected or another V-pulse is delivered. The ventricular Maximum Sensitivity thereby represents the maximum permissible sensitivity of the ventricular sense amplifier (or the minimum permissible sensitivity mV value).
Once another R-wave is detected (or another V-pulse is delivered), the ventricular sensitivity mV value is then bumped up to the Starting Sensitivity value (thus making the ventricular sense amplifier again less sensitive) and the process repeats. Additionally, ventricular refractory (VREF) intervals are activated after initial detection an R-wave. During VREF, any signals sensed by the ventricular sense amplifier are ignored, at least for the purposes of triggering or inhibiting pacing pulses. The signals can nevertheless be examined and recorded for diagnostic purposes. Separate VREF values are used following R-waves and V-pulses. A similar ASC procedure may be performed for the atrial sense amplifier to automatically adjust an atrial sensitivity following detection of an atrial depolarization event (also referred to as a P-wave) or delivery of a atrial pacing pulse (i.e. an A-pulse.)
Hence, during ASC, the device automatically adjusts the atrial and ventricular sensitivity values during each cardiac cycle (i.e. each heartbeat.) This is performed to enhance the specificity with which atrial and ventricular events are detected. In particular, on the ventricular channel, the ventricular sensitivity mV value is initially kept at a relatively high level following each R-wave/V-pulse to prevent the corresponding T-wave (i.e. ventricular repolarization) from being erroneously detected and misinterpreted as another R-wave (which is referred to as “oversensing”). The ventricular sensitivity mV value is ultimately lowered to help ensure that the next R-wave of the next cardiac cycle is properly detected. For the atrial channel, the atrial sensitivity mV value is initially kept at a relatively high level following each P-wave/A-pulse to prevent the far-field R-waves (i.e. R-waves generated in the ventricles but sensed in the atria) from being erroneously detected and misinterpreted as P-waves. The atrial sensitivity mV value is ultimately lowered to help ensure that the P-wave of the next cardiac cycle is properly detected. By helping to prevent erroneous detection of T-waves, far-field R-waves, etc., ASC thereby helps ensure that atrial and ventricular rates are reliably detected so that pacing or defibrillation therapy, if need, can be reliably and appropriately delivered. (Note that, strictly speaking, P-waves, R-waves and T-waves are features of the surface electrocardiogram (EKG). For convenience and generality, the terms P-wave, R-wave and T-wave are used herein to refer to the corresponding internally-detected signal components, i.e. the corresponding components of the IEGM. The amplitudes and widths of these features and the intervals therebetween are also referred to herein as morphological features of the cardiac signal.)
FIG. 1 illustrates a set of exemplary ventricular ASC parameters 2 in connection with stylized ventricular cardiac signals 4 associated with intrinsic ventricular depolarizations. Atrial ASC parameters are generally similar, but are defined relative to P-waves (which are not shown in FIG. 1). Exemplary ventricular ASC values are as follows. Starting Sensitivity may be programmed to 50% of present R-wave amplitudes. The ventricular post-sensed and post-paced Decay Delay values may both be set to 60 milliseconds (ms). The ventricular Decay Rate may be set to 3 mV/second. The ventricular Maximum Sensitivity may be set to 0.3 mV. VREF(sensed) may be set to 125 ms). VREF(paced) may be set to 250 ms. Atrial values typically differ from their corresponding ventricular values. For example, atrial post-sensed and post-paced atrial Decay Delay values may both be set to 0 ms. The atrial Decay Rate may be set to 1.5 mV/second. The post-sensed atrial refractory period (AREF) may be set to 93 ms). AREF(paced) may be set to 190 ms. With current devices, the ventricular and atrial ASC parameters are programmed by a physician or other clinician using an external programmer device following device implant. Thereafter, the ASC parameters are not changed until a follow-up programming session. That is, the various ASC parameters are fixed parameters that are not automatically adjustable by the device itself. Moreover, although ASC controls sensitivity in response to changes in R-wave amplitude (by setting the Starting Sensitivity mV value based on a percentage of R-wave amplitude), ASC does not take into account changes in T-wave amplitude.
Hence, although ASC is a very effective technique for automatically adjusting sensitivity throughout each cardiac cycle, there is room for further improvement. In particular, cardiac signal amplitudes, including T-wave amplitudes as well as R-wave and P-wave amplitudes, may fluctuate depending on lead maturation and fibrosis, lead micro-dislodgement, patient activity, drug intake and a variety of other factors, such that ASC parameters specified by the physician may no longer be appropriate. In this regard, most physicians perform a sensing threshold test during a follow-up session with the patient and then set the ASC parameters accordingly. Unfortunately, the selected parameters may not be appropriate during subsequent episodes of cardiac signal instability. As such, if the resulting sensitivity set by the ASC procedure is too low, some cardiac events will not be detected. If the sensitivity set by ASC is too high, erroneous sensing of noise may occur or undesired cardiac signals may be incorrectly classified. Sensing problems can potentially result in abnormal/inappropriate device function, inappropriate therapy, incorrect diagnostic data collection, inappropriate arrhythmia detection and inappropriate mode switching.
Inappropriate sensing may occur in a patient in many forms. For example, over-sensing on the ventricular channel can cause T-waves to be erroneously counted as R-waves. Over-sensing on the atrial channel can cause far-field R-waves to be mistaken as P-waves. Noise over-sensing on either channel (possibly resulting from diaphragmatic myopotentials, skeletal myopotentials, etc.) can cause noise to be mistaken as P-waves or R-waves. Under-sensing may arise due to a decrease in P-wave or R-wave amplitudes, where the amplitude variation may result from respiration effects, exercise, or the presence of an arrhythmia. In particular, atrial arrhythmia typically causes a reduction in P-wave amplitude to as little as ⅕th of the true P-wave amplitude. Other factors such as drugs or electrolyte imbalances can cause inappropriate sensing. In addition, patients with cardio-myopathies are more prone to sensing malfunctions. Several other factors may also affect the device's sensitivity over a period of time, such as fibrotic tissue growth, lead micro dislodgment, lead fracture and changes due to defibrillation shock.
Thus, the programmable ASC parameters may need to be frequently adjusted by the physician to compensate for noise or other factors. However, patients are typically not monitored by the physician more frequently than once every six months; and often less frequently. Hence, the implanted device may operate incorrectly (or at least sub-optimally) for six months or more before the ACS parameters are adjusted by the physician.
Some techniques have been proposed that appear to allow a Maximum Sensitivity-type parameter to be automatically adjusted by the device based on a detected noise floor. See, e.g., U.S. Patent Application 2006/0085038 of Linder et al., entitled “Method And Apparatus For Adjusting Cardiac Event Detection Threshold Based On Dynamic Noise Estimation”; U.S. Patent Application 2003/0097157, of Wohlgemuth et al., entitled “Implantable Medical Device With Autosensitivity Algorithm For Controlling Sensing Of Cardiac Signals”; and U.S. Patent Application 2004/0106957 of Palreddy et al., entitled “Method and System for Noise Measurement in an Implantable Cardiac Device.” Adjusting the Maximum Sensitivity value based on a noise floor helps address some sensing problems that arise due to variations in signal channel noise. However, other sensing problems, particularly those arising due to changes in the morphology of the cardiac signal, are not corrected merely by taking noise into account. Moreover, the techniques described within the aforementioned patent applications are not necessarily applicable to ASC, which, as explained, is a particular type of sensitivity adjustment technique that employs a particular set of programmable parameters. Also, it does not appear that the predecessor techniques specifically detect the sizes and shapes of T-waves for the purposes of adjusting ASC parameters so as to optimize R-wave detection while avoiding T-wave oversensing.
Accordingly, it would be highly desirable to provide techniques for remedying problems that arise in devices equipped to perform ASC and it is to that end that the invention is primarily directed.